The present invention relates to electronic ballasts for powering high-luminance lamps such as a high-pressure mercury lamp or metal halide lamp.
FIG. 21 is a circuit diagram of a conventional electronic ballast for a discharge lamp DL. FIG. 22 is a waveform chart illustrating the operation of the ballast of FIG. 21. The detailed configuration and operation of the ballast are described later in connection with the description of FIG. 2 and FIG. 3. Only a summary outline of the operation is described here.
When the high pressure discharge lamp DL is not illuminated, the ballast operates in a starting mode to start the lamp DL. In the starting mode, a step-down chopper circuit 12 outputs a DC voltage which is higher than the voltage which is applied when the lamp DL is in steady-state operation. The DC voltage is converted into a rectangular wave AC voltage by inverter circuit 13, and the rectangular wave AC voltage is applied to the lamp DL via a starting voltage generating circuit 2. When the polarity of the rectangular wave AC voltage is inverted, a voltage responsive switching element Q7 turns on and the starting voltage generating circuit 2 generates a high lamp starting voltage. When the starting voltage is applied to the lamp DL, dielectric breakdown occurs in the lamp and a glow discharge takes place.
After shifting to an arc discharge state, lighting detector circuit 31 detects the fact that the lamp DL has been started. The ballast operation is thereby shifted to a lighting mode. Then, an output detector circuit 12a detects an output voltage of the step-down chopper circuit 12, and a chopper control circuit 12b controls the operation of the step-down chopper circuit 12 so that a predetermined current corresponding to the output voltage can be provided, whereby rectangular wave-shaped appropriate power is supplied to the lamp DL via the inverter circuit 13 and the lamp DL lights in a steady-state. The temperature and pressure in the light-emitting tube of the lamp DL gradually increases after start-up, and the lamp DL transitions into a steady lighting state.
When the lamp DL is extinguished for some reason, the lighting detector circuit 31 detects the non-lighting state and determines that the lamp DL has extinguished. In such a case, to start the lamp DL again, a high starting voltage is generated and applied to the lamp DL by the starting voltage generating circuit 2. It is known that because the temperature and pressure in the light-emitting tube are very high when the lamp DL extinguishes from a steady lighting state, the dielectric breakdown voltage in this state can be as high as a few dozen kV. To restart the lamp DL immediately in this state, it is necessary to apply a voltage of a few dozen kV as described above to the lamp. This can cause problems of how to safely insulate the lamp fixture and wiring and can place high stresses on the lamp.
For example, in the case of an ordinary lamp having an Edison base 23 as shown in FIG. 23, when a voltage of a few dozen kV is applied, dielectric breakdown occurs in the Edison base 23 or in an area designated as A in an outer tube 22. As a result, the necessary voltage is not applied to the light-emitting tube 21. Therefore, ordinary ballasts are designed to generate and apply a dielectric breakdown voltage for a lamp of interest in a sufficiently cooled state (approximately 4 kV, for example) at start-up. Thus, when the lamp is extinguished from a steady lighting state, it cannot be immediately restarted. The lamp can be restarted when the temperature in the light-emitting tube decreases and the dielectric breakdown voltage decreases to the high starting voltage that is generated by the starting voltage generating circuit 2 or lower.
As described above, immediately after a lamp extinguishes from a steady lighting state, the temperature and pressure in the light-emitting tube are high and the dielectric breakdown voltage is very high. The output of the ballast in this state is generally the same as that in a no-load state in which the lamp is not connected. Because it is not easy to distinctly detect the no-load state and the state immediately after the extinguishing of the lamp, a high starting voltage is generated even in a no-load state. In addition, even when the lamp is at the end of its useful life and a starting failure occurs, the ballast generates a high starting voltage.
When a high starting voltage is generated over a long period of time in such a no-load state or at the end of useful life of a lamp, the ballast may be damaged. Therefore, some prior art ballasts use a technique in which, in the case of a lamp start failure, generation of a high starting voltage is repeated up to a predetermined number of times and the generation of a high starting voltage is stopped when the predetermined number of times is reached.
Moreover, when a lamp is extinguished from a steady lighting state, it may take ten minutes or more to restart it, depending on the type of the lamp and the structure and the installation conditions of the fixture. Even when a high starting voltage is continuously generated during that time, the lamp cannot be started and a high voltage of about 4 kV, for example, continues to be applied between the lamp terminals. Because this wastes electric power and can be unsafe, some have proposed generating a high starting voltage intermittently.
The possible reasons for extinguishing of an HID lamp include power shutdown (such as switched shutdown or power failure) and brief power fluctuations such as instantaneous power failure or instantaneous voltage drop. When a power shutdown occurs, the control circuit is reset unless it has a non-volatile memory or the like because no power is supplied to the ballast for a relatively long period of time. Therefore, an operation to restart the lamp must be performed after the restoration of the power source.
On the other hand, when a brief power fluctuation such as instantaneous power failure or instantaneous voltage drop occurs, the lamp is extinguished but the operation of the control circuit is maintained by an electric charge accumulated in a capacitor for a control power source. If the number of start failures during the process of starting the lamp is maintained as the operation information of the control circuit as in some prior art ballasts, when the lamp is restarted after restoration of the power, the predetermined number of start failures is reached before a starting voltage is applied over the time necessary for restart, and the operation is stopped. In other cases, if the generation cycle of the intermittent high starting voltage is maintained as in other prior art ballasts, a failure occurs in which the operation is started from a quiescent period in the intermittent high starting voltage generating period and the high starting voltage cannot be immediately applied to the lamp.
In other prior art ballasts, a technique is proposed in which a detection circuit for detecting a change of a lamp from a lighting state to a non-lighting state is provided. The information during the previous starting process is reset when the fact that the lamp has extinguished is detected so that a high starting voltage can be applied immediately after the lamp is extinguished, and a sufficient time to apply a starting voltage necessary for restart can be insured.
In this prior art technique, when the lamp shifts from a lighting state to a non-lighting state, the information acquired during the previous starting process is reset. However, when the lamp extinguishes after it has been lighted for a short period of time before reaching a steady lighting state, there are cases where it is undesirable to reset the information acquired during the previous starting process.
For example, when an end-of-life lamp is connected, a phenomenon is known in which, after a dielectric breakdown is caused by the starting voltage and a glow discharge is formed, the lamp is extinguished after a brief transition to an arc discharge. In such a case, when the operation information during the starting process is reset upon detection of the fact that the lamp is extinguished, the operation information acquired during the starting process is reset every time the lamp is extinguished. Thus, a high starting voltage is applied every time the lamp is extinguished, and a failure occurs in which the cycle consisting of dielectric breakdown, glow discharge, arc discharge, lamp extinguished, and application of a high starting voltage continues endlessly and generation of the starting voltage cannot be stopped.
The present invention has been made in view of the above circumstances, and it is, therefore, an object of the present invention to provide a ballast which can prevent generation of an unnecessary high starting voltage to ensure power saving and safety even when a lamp fails to start at, for example, the end of useful life, and which can provide a lamp with reliable starting and restarting characteristics in normal cases.